1 files changed, 50 insertions, 0 deletions
diff --git a/engine/sine.asm b/engine/sine.asm
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+++ b/engine/sine.asm
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+_Sine:: ; 84d9
+; A simple sine function.
+; Return d * sin(e) in hl.
+
+; e is a signed 6-bit value.
+	ld a, e
+	and %111111
+	cp  %100000
+	jr nc, .negative
+
+	call .ApplySineWave
+	ld a, h
+	ret
+
+.negative
+	and %011111
+	call .ApplySineWave
+	ld a, h
+	xor -1
+	inc a
+	ret
+
+.ApplySineWave: ; 84ef
+	ld e, a
+	ld a, d
+	ld d, 0
+	ld hl, .sinewave
+	add hl, de
+	add hl, de
+	ld e, [hl]
+	inc hl
+	ld d, [hl]
+	ld hl, 0
+
+; Factor amplitude
+.multiply
+	srl a
+	jr nc, .even
+	add hl, de
+.even
+	sla e
+	rl d
+	and a
+	jr nz, .multiply
+	ret
+
+.sinewave: ; 850b
+; A $20-word table representing a sine wave.
+; 90 degrees is index $10 at a base amplitude of $100.
+	sine_wave $100